implement compress option reduce_funcs (#2466)

- inline single-use function declarations as expressions when permissible
- depend on `reduce_vars`
- enabled by default
- disable for speed critical code

fixes #2464

test/compress/issue-1656.js

@@ -15,6 +15,7 @@ f7: {
         negate_iife: true,
         passes: 3,
         properties: true,
+        reduce_funcs: true,
         reduce_vars: true,
         sequences: true,
         side_effects: true,